That Differential GPS stuff

To amplify a bit on my last posting (if you're not
interested, delete now),
Or, yet another use for satellites.
Differential GPS is a way of partially overcoming GPS's
selective availability - an intentional scrambling of range
information to limit just how accurate a fix non-military
users can get with GPS. Typical SA accuracy is on the order
of 100 meters. Any more accurate and GPS becomes useful for
unfriendly people to perform unfriendly acts, if you
get my drift...
The net result is that GPS fixes tend to be fuzzy, producing
a cloud of fuzzy pseudo-fixes around where you really are.
Differential GPS is one answer to this. A differential GPS
beacon knows where it is, finds out where GPS says it is,
and broadcasts the difference. If you're reasonably close
to the DGPS beacon, the difference is applicable to you too.
Feed this correction information to a GPS receiver and you
get much improved accuracy.
DGPS beacons transmit their data (100 or 200 baud) around
300 kHz, a frequency range already used for marine beacons -
the U.S. Coast Guard have been heavily involved, being
interested in things like navigating ships around harbours
and coastlines.
One person has already posted a DGPS receiver to the web
(http://web.abnormal.com/dgps/bix/ ), but I'm thinking
something a little more basic (direct conversion, analogue
tuning) to start with.
I'm also curious about the Russian navigation satellites
I hear going over (listen for data around 150 MHz) and
if I can extract any useful information from them.
Laura Halliday VE7LDH "Laisse le vent tout emporter..."
Grid: CN88hk IOTA: NA036 - Foly/Viennet
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